In a piston engine, the initial injection of water cools the fuel-air mixture significantly which increases its density and hence the amount of mixture that enters the cylinder. The water (if in small liquid droplets) may absorb heat (and lower the pressure) as the charge is compressed, thus reducing compression work.
[1] An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, and reducing the amount of heat energy absorbed into the cylinder walls. This also converts part of combustion energy from the form of
heat to the form of
pressure. As the water droplets vaporize by absorbing heat, it turns to high pressure steam (water vapor or steam mainly resulted from combustion chemical reaction). The alcohol in the mixture burns, but is also much more resistant to
detonation than
gasoline. The net result is a
higher octane charge that will support very high compression ratios or significant forced induction pressures before onset of detonation.
Fuel economy can be improved with water injection. Depending on the engine, the effect of water injection, with no other modification, like leaning out the mixture, may be quite significant
[1] or rather limited and in some cases negligible[
original research?].
It should be noted that most modern consumer vehicle engines are pre-programmed with specific fuel-to-air ratios and that introducing water without re-programming the car's computer will not have any benefit and in most cases will reduce performance and possibly damage the engine. Most modern fuel systems cannot determine that water in any form has been added and cannot determine a new compression ratio or take advantage of lower cylinder temperatures. In most cases in cars that are pre-programmed, introducing water vapor will cause them to lose power because the water vapor will then take the place of air and fuel that is required to produce power. Only in vehicles in which the owner can take the car to a
tuner will any benefit be seen. However, as soon as the water source is depleted the engine will be running rich and power and and fuel economy will be lost again.
Damage can occur to the engine if too much water is injected or if there is a malfunction of the injector itself. Water is not compressible and if too much water makes it into the cylinder prior to detonation (during the compression cycle), the volume of liquid may exceed the engine block's head bolts threshold, causing
blown heads,
hydrolock, or shatterting of the piston connecting rods. This damage is fatal to the engine and typically requires a full rebuild.
Water injection is typically used in aviation and is not intended for use in consumer grade vehicles. When used in consumer grade vehicles it is for the extremely high performance arena (1000 HP+) where vehicles are super- or turbocharged and produce excessive heat. These vehicles are typically driven only in a event or controlled race and require extremely high levels of knowledge to maintain and are only driven for very short distances. In most forms of water injection, water is mixed with a combustible fuel and not injected directly from the tap.
Many product scams exist[
citation needed] touting that inserting a tube of misting spray into a intake manifold will increase your mileage gains or horsepower. However, unless the user reprograms the car's
ECU, power loss and damage will eventually occur.
